1. Field of the Invention
The present invention relates to a novel fluorine-containing polyimide and more particularly relates to a thermoplastic polyimide which comprises fluorine, is colorless and has high transparency and very low dielectric characteristics, and a preparation process of the same and a novel aromatic diamino compound which can be used for preparing the thermoplastic polyimide.
The novel aromatic diamino compound of the invention is useful as a raw material of polyimide of the invention and can also be applied to a raw material of other kinds of polyimide, polyamide, polyamideimide, bismaleimide and epoxy resin.
2. Related Art of the Invention
Polyimide resins obtained by reaction of tetracarboxylic dianhydride and diamine have mechanical strength, dimensional stability, flame retardance and electrical insulation properties in addition to high heat resistance, and thus have been conventionally used for raw materials or heat resistant adhesives in the field of electric and electronic appliances, space and aeronautic equipment and transportation machinery.
For example, as a product of the polyimide resins, polyimide films are expected to be widely used due to their excellent properties in the field where heat resistance is required.
In recent years, polyimide resins having a higher level of heat resistance and adhesive strength have been developed in order to use under more harsh conditions. Many kinds of polyimide which have been conventionally developed exhibit excellent properties. However, conventional polyimide is generally poor in light transmittance and has a strong tendency to color dark yellow or brown. The transmittance is particularly low in the visible region and the color has a defect of becoming deeper in the outer space. On the other hand, as to resins which are colorless and transparent or almost colorless and transparent in hue, a polyester film, polycarbonate film and aliphatic polyimide film have been developed. These films, however, are inferior in heat resistance and weatherability over a long period.
However, in the field of space development of recent years, a film having low absorption to cosmic rays has been demanded as a cover lay of a solar cell and thus development of a resin which has heat resistance and is colorless and transparent is strongly desired. Additionally, coloration is a very serious problem in the development of optical instruments such as cables and filters in an optical-fiber telecommunication system and liquid crystal display boards which require high heat resistance and high reliability.
Transparence of marketed polyimide is practically indicated, as a parameter, by an yellowness index (hereinafter referred to simply as YI) which is an indicator of yellow hue. Any marketed polyimide has very high YI. For example, Kapton (Trade Mark) prepared from 4,4'-diaminodiphenyl ether and pyromellitic dianhydride has YI of 129, polyimide Upilex-S (Trade Mark) prepared from p-phenylenediamine and biphenyltetracarboxylic dianhydride has YI of 125, and polyimide LARC-TPI (Trade Mark) prepared from 3,3'-diaminobenzophenone and benzophenonetetracarboxylic dianhydride has YI of 50.
In order to use for various optical instruments above, YI must be 10 or less. Polyimide resin comparable in quality to polycarbonate which is at present widely used for optical instruments has desirably YI of 7 or less.
As colorless and transparent polyimide, Japanese Laid-Open Patent SHO 63-170420 has already disclosed a polyimide homopolymer having recurring structural units represented by the general formula (A): ##STR2## or a polyimide copolymer having recurring structural units represented by the general formula (A) and those represented by the general formula (B): ##STR3##
Such polyimide has very excellent transparence. However, essential heat resistance of polyimide is insufficient.
On the other hand, microelectronics has remarkably developed in recent years in the electric and electronic field. High speed transmission of signals becomes inevitable in a large-size computer in particular as a result of employing a multi-layer circuit substrate. However, a high dielectric constant of the substrate material leads to delay transmission of signals and causes problems on speeding-up the responses. Polyimide is used for an interlayer insulation membrane in a multi-layer wiring structure. Particularly the need for decreased dielectric constant has been closed up in addition to the above characteristics of conventional polyimide.
Teflon is a conventionally known fluorine-containing resin and has been known for a long time to have a low dielectric constant. Further, the fact that the dielectric constant of the polyimide resin can be lowered by introducing a fluorine atom or fluoro radical into the polyimide structure, has been reported, for example, in A. K. St. Clair et al., Polymeric Materials Science and Engineering, 59, 28.about.32 (1988), and EP 029986.
However, presently marketed polyimide resins have a dielectric constant of 3.5/1 MHz in Kapton, 3.5/1 MHz in Upilex-S, and 3.7/1 MHz in LARC-TPI. Conventional polyimide containing a large amount of fluorine is difficult to manufacture in industry because a polymer of sufficiently high molecular weight cannot be obtained or production cost is very high as described, for example, in Japanese Laid-Open Patent HEI 1-182324, 2-60933, 2-281037 and 4-122729. Consequently, it has been strongly desired to develop polyimide resin having a high molecular weight and a price as low as possible.
Any problem above results from electron transfer in the resin. Cutting of a conjugated system in the main chain of a polymer can improve the coloration and restriction of electron transfer in the main chain of the polymer can decrease the dielectric constant. In practice, it has been known that introduction of an electron-attractive fluorine atom into molecular units of polyimide is effective for solving these problems. For example, aromatic diamine compounds comprising a hexafluoroisopropylidene radical have been disclosed as a monomer of polyimide for low dielectric material in Japanese Laid-Open Patent HEI 1-190652. These aromatic diamine compounds, however, have problems in industry and in physical properties. For example, these compounds must be prepared by multi step synthesis or polyimide resin derived from these compounds is lacking in melt flowability for processing the resin.
As to diamine having a trifluoromethyl radical in the molecule, for example, 4,4'-bis(4-amino-5-trifluoromethylphenyloxy)biphenyl of the formula (C): ##STR4## has been disclosed in DE 3,490,169.
However, the electron attractive trifluoromethyl radical of the compound is on a terminal aromatic ring where an amino radical is present and locates at an ortho-position to the amino radical.
Consequently, it has been known that the compound is difficult to react with acid anhydride due to an electronic factor and a high molecular weight polymer is difficult to obtain. Additionally, the amino radical is situated at the para position to an ether bond, and thus resultant polyimide has a rigid structure and also has a defect of difficulty on processing. Improvement of adhesion is also desired.